Sudan suffers from years of vegetation degradation and is also hit by climate change which has fatal consequences on its fragile economy and the lives of its 41 million people. The vegetation-soil moisture relationships describe the various vegetation patterns which occur in the country. Natural regeneration of Sudan’s vegetation remains the only possible solution for combating this degradation and ultimately contributing to the country’s economic and social stability. In light of these facts the current thesis tries to understand the physical circumstances that impact vegetation regeneration via studying the connections between soil water, erosion and some of the main elements of the hydrological cycle, namely evapotranspiration, temperature and rainfall. The studies of soil moisture and its climatic associations in Sudan are rare especially in the thorough way that is presented here. These results have important food security implications, informing agricultural development, environmental conservation, and water resource planning.

The first stage of this research sought to evaluate the spatial distribution of soil erosion as one of the implications of soil degradation which poses a serious environmental and socioeconomic threat to the environment and to mankind. The developed erosion model used multispectral Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) and Moderate Resolution Imaging Spectroradiometer (MODIS) products from March and December 2006 plus a Shuttle Radar Topography Mission (SRTM) digital elevation model. The results allowed the identification of erosion gullies and subsequent estimation of eroded area. River flow network and slope are identified as more important natural factors, in causing erosion in the Blue Nile region than aspect and elevation.

The second stage concentrated on evaluating evapotranspiration as a direct reflection of the dynamics of soil moisture and hence vegetation regeneration. Both potential and actual evapotranspiration vary from day to day and have a seasonal cycle that determines the rates of vegetative growth and water stress. This thesis, among other, compares three methods for the estimation of daily actual evapotranspiration in Blue Nile. The methods are the remote sensing using the satellite based Surface Energy Balance Algorithm for Land (SEBAL) model with Moderate Resolution Imaging Spectroradiometer (MODIS) satellite data, the modified Thornthwaite water balance method, and the complementary relationship method. A sequence of spatial distribution maps of seasonal actual evapotranspiration are produced. From the maps it was concluded that in the dry season, the spatial distribution pattern is determined by the location, aspect, land use and irrigation activities. In the wet season, the spatial distribution pattern followed that of the rainfall distribution. The seasonal patterns of actual evapotranspiration are a result of the combined effect of rainfall and soil moisture. The seasonal patterns of monthly soil moisture followed those of the monthly rainfall, but with about one month delay in the phase.

The third stage aimed at better understanding the soil moisture variability as a fundamental factor for vegetation regeneration during the period 1965-2005. NCEP/NCAR and PERC reanalysis data are used to study the general trend and to understand the soil moisture, temperature and rainfall relations using Mann Kendall test and geographically weighted regression. To further understand dry and wet variations in terms of regeneration demand, the aridity index is used. The results showed that there are decreasing trends of soil moisture on an annual and seasonal level and that the trend is less dramatic or weaker in the dry season (November-April) than the wet season (May- October). The long-term average of aridity index is affected by the reported decline in rainfall during 1965-1985.

Paper II El Tahir, M., Wenzhong, W., Xu, C., Youjing, Z., and Singh, V. (2012). Comparison of methods for estimation of regional actual evapotranspiration in data scarce regions: The Blue Nile region-Eastern Sudan. Journal of Hydrologic Engineering, v17, nr 4, pp 578–589. The paper is removed from the thesis in DUO due to publisher restrictions. The published version is available at: https://doi.org/10.1061/(ASCE)HE.1943-5584.0000429

Paper III M. El Haj El Tahir, C-Y Xu, Zengxin Zhang (2012). Soil moisture characteristics and implication for vegetation regeneration in Sudan during the period 1965-2005. Submitted to Journal of Stochastic Environmental Research and Risk Assessment. The paper is removed from the thesis in DUO due to publisher restrictions.

Paper IV Chong-Yu Xu, Qiang Zhang, M. El Hag El Tahir, Zengxin Zhang (2010) Statistical properties of the temperature, relative humidity, and net solar radiation in the Blue Nile-eastern Sudan region. Journal of Theoretical and Applied Climatology, V101, nr 3-4, pp 397-409. The paper is removed from the thesis in DUO due to publisher restrictions. The published version is available at: https://doi.org/10.1007/s00704-009-0225-7